The invention relates to an illumination optical system for endoscopes, and more particularly, to a system which is adapted to provide a uniform illumination to both near and far objects for purpose of observation by an endoscope.
Endoscopic diagnosis has become increasingly important in the early detection of cancers such as those which are found in the stomach. An endoscope which is used for such medical diagnosis is illustrated in FIG. 1 where its distal end 1 adapted to be inserted into coeloma is shown. A cover glass 2 is disposed in the end face of distal end 1 for passing light from an illumination optical system in order to illuminate an effected area within the coeloma. Another cover glass 3 is also disposed in the end face and is associated with an observation optical system which is used to observe an affected area that is illuminated by the light emitted through cover glass 2. In addition, channels 4, 5 are shown disposed in the end face to permit an access of forceps, for example, that is used to treat the affected area.
Referring to FIG. 2, light guide 6 is shown to be disposed adjacent cover glass 2 and is contained within the distal end of the endoscope. Light guide 6 has its end face 6a disposed close to cover glass 2 so that the illuminating light which is emitted from end face 6a passes through the latter. Image guide 7 is disposed parallel to light guide 6 within the distal end, and has its end face 7a located in opposing relationship with cover glass 3 with observation lens 8 interposed therebetween. Observation lens 8 is movable between cover glass 3 and image guide 7 for purpose of focus adjustment.
During the observation of a coeloma with an endoscope, it is sometimes necessary to observe the entire situation including an affected area and its surrounding regions, by placing the distal end of the endoscope at a distance from the affected area which is to be observed. At other times, it is also necessary to bring the distal end of the endoscope into close proximity to the affected area so that an enlarged image of the affected area or a desired limited region can be obtained. In normal use, the distal end of the endoscope is continually moved close to or away from the affected area while the diagnostician is observing the affected area. In these situations, it is undesirable for the image to vary as a function of the distance between the distal end of the endoscope and an affected area, which is illustrated in FIG. 2 by far object 11 and near object 12. Rather the illumination system should provide a uniform, high level illumination covering both far and near located objects. However, with a conventional endoscope which involves a parallax between the observation and the illumination system, it will be seen that when distal end 1 is brought into proximity to near object 12 such that observation cover glass 3 almost contacts an area P12 of object 12 which is to be observed, as shown in FIG. 2, the presence of cover glass 3 which is located opposite to area P12 prevents enough light from reaching such area to assure an accurate observation.
To overcome the difficulty which results from the parallax, it has been proposed to dispose concave lens 9 (FIG. 3) between the end face 6a of light guide 6 and cover glass 2 to permit a diffusion of light emitted by the end face 6a so that enough light can be projected to area P12 if cover glasses 2, 3 are located close to near object 12.
However, with the illumination optical system illustrated in FIG. 3, when the distal end of the endoscope is brought to a position remote from the object to illuminate an increased area, the amount of light reaching far object 11 will be substantially reduced. Consequently, the arrangement which attempted to provide a uniform illumination of far object 11 and near object 12 resulted in a minimized illumination of far object 11 at this position of the distal end of the endoscope, rendering the observation difficult.
As indicated in broken lines in FIG. 3, when the light which passes through the central region of concave lens 9 illuminates a marginal area Q12 surrounding an area P12 to be observed, and which is located opposite cover glass 2, the mirror action of the surface of marginal area Q12 reflects the light, causing it to impinge on the observation optical system through cover glass 3. Such impinging light will be reflected as by the inner surface of frame 10 associated with lens 8, to be incident on end face 7a of image guide 7, creating a ghost. The light which impinges in this manner will be repeatedly reflected in an intricate manner between lens 8 and frame 10 before it reaches the eyes of an observer through image guide 7, producing a flared field of sight, and preventing a normal observation. In particular, when concave lens 9 is used to provide a wide angle of illumination as illustrated in FIG. 3, a relatively intense light which passes through the central region of guide light 6 and illuminates the marginal area Q12 of the close object 12 will produce, by reflection, a ghost having a brightness level which is substantially greater than that of light which passes through the marginal region of light guide 6 and strongly refracted by concave lens 9 to impinge on area P12 to be observed of the same object 12. This means that an image being observed which is of a reduced level is superimposed with a flare of higher brightness. This is a disadvantage since it renders the observation of the image particularly difficult.
To overcome the described disadvantage which results from the use of concave lens 9 shown in FIG. 3, there has been proposed a different arrangement shown in FIG. 4 in which a second illumination optical system is provided which comprises second cover glass 13 disposed at a different angle from that of cover glass 2, and second light guide 14 having its end face 14a disposed close to cover glass 13 for supplying an illuminating light thereto in order to provide a satisfactory illumination of area P12 of the near object 12, which is particularly susceptible to the influence of the parallax. This arrangement enables a uniform illumination of an increased level for both far object 11 and near object 12, overcoming the described disadvantage. However, because the pair of illumination systems must be used for purpose of illumination, it becomes necessary to increase the thickness of the distal end of the endoscope which is to be inserted into the coeloma. Since it is otherwise preferable to form the distal end of the endoscope with a reduced thickness, this solution is not entirely satisfactory. An additional drawback of this arrangement is that it increases the number of parts, resulting in a complex structure and added adjustment difficulty.